Peierls Transition Research Articles

Mo-edge reconstructions in MoSe2 and MoS2 : Reexamination of the mechanism

For zigzag nanoribbons of transition metal dichalcogenides (TMDCs), experiments show a band gap with different gap size at the Mo edges, while theoretical models often predict metallic edges. Such an inconsistency could be attributed to the inadequate understanding of the possible mechanisms of edge reconstructions. Here, we revisit the mechanisms of different edge reconstructions along the Mo edge of $\mathrm{Mo}{X}_{2}$ ($X=\mathrm{Se}$, S) zigzag nanoribbons by first-principles calculations. We demonstrate that the valency of $\frac{1}{3}$ edge Mo increases to 5+ with $X$ adatom reconstruction, or all edge Mo decreases to 3+ with a Mo trimer reconstruction. Based on these Mo valence reconstructions, thermodynamically stable reconstructions are proposed for the Mo edge in $6\ifmmode\times\else\texttimes\fi{}$ and $4\ifmmode\times\else\texttimes\fi{}$ periodicities, which are comparable with the recent experimental observation of a small energy gap (of 0.36 eV) at the Mo edge of zigzag $\mathrm{Mo}{\mathrm{Se}}_{2}$ nanoribbons. The edge Mo valency reconstruction leads to quasi-one-dimensional Peierls distortion and charge- and spin-density waves at the edge. These findings should be applicable to other TMDCs.

Collective electronic excitations in charge density wave systems: The case of CuTe

The study of neutral electronic excitations directly probed by electron energy loss spectroscopy experiments allows obtaining important insight about the physical origin of the charge density wave (CDW) transition in solids. In particular it allows us to disentangle purely phononic mechanisms from the excitonic insulator scenario which is associated to a purely electronic mechanism. As a matter of fact, while the the loss function of the excitonic insulators should display negative dispersive features associated to the softening of neutral electronic excitations at the CDW wave vector above the critical temperature, no softening is expected when the driving force is purely phononic. Here we perform a microscopic analysis of the dynamical charge response of CuTe, a material that displays a low-temperature Peierls-like CDW instability. By means of first-principles time-dependent density functional calculations of the loss function, we characterize the plasmon dispersion along the different directions, highlighting the role of the intrinsic structural anisotropy and the effects of the crystal local fields that are responsible for the periodic reappearance of the spectra of the first Brillouin zone as well as the formation of an acousticlike plasmon. Finally, we demonstrate that also in this system, in analogy with other materials displaying excitonic insulator instability, the low energy region of the loss function presents negative dispersive structures at momentum transfer close to the CDW wave vector. This is a feature common to both excitonic insulator transition and Peierls distortion that further highlights how the difference between the two mechanisms is at most quantitative.

Metal-insulator transition in Cr-doped hollandite vanadate K2V8O16

The hollandite K2V8O16 is a metal with ferromagnetic spin fluctuations in the high temperature (tetragonal I4/m) phase and encounters metal-insulator transition (MIT) at 170 K. The mechanism behind the MIT is still controversial due to inadequate information on the electronic structure. Furthermore, substitution of Rb for K or Ti for V increases the transition temperature. We have investigated whether break in the mirror symmetry is responsible for the insulating ground state of K2V8O16 and since the Cr counterpart is a robust ferromagnet, whether Cr doping which leads to break in mirror symmetry can help in driving a gap. We find that both 25% and 75% Cr doping break the mirror symmetry but only the former leads to an insulating ground state. Ferromagnetism (FM) remains intact in the insulating phase of K2V6Cr2O16. The structural, electronic and magnetic properties of pure and doped K2V8O16 were investigated within first-principles calculations using density functional theory (DFT). Electron correlation suppresses orbital fluctuations between the partially occupied Cr and V-3dt2g states. Consequently, transfer of charge (electron) from V-3d to Cr-3dt2g states is observed which facilitates Cr3+-V4+ charge ordering. Furthermore, Peierls like structural distortion is associated to the breaking of mirror symmetry in the Cr-V rectangular four chain columns within the crystal. Therefore, the simultaneous effect of Peierls instability, charge ordering and Coulomb correlation is responsible for the MIT in K2V6Cr2O16. Besides, Cr-3d O-2p hybridization and Cr-O coupling increases with Cr doping. These two effects are cooperatively responsible for the observed FM in insulating K2V6Cr2O16. More interestingly, the strength of FM is augmented with Cr concentration in K2V8O16.

Nature of optical excitations and bandgap of Re x Mo1−x S2 alloy at nanoscale probed from high resolution low loss electron energy loss spectroscopy

The two-dimensional (2D) transitional metal dichalcogenides (TMDS) have become an intensive research topic recently. The alloys of these TMDs have offered continuous tunability of the bandstructure and carrier concentration, providing a new opportunity for various device applications. Here the rich variations in optical excitations in Re x Mo1−x S2 alloy at the nanoscale region are shown. The alloy bandgap and charge response are probed by low-loss high-resolution transmission electron energy loss spectroscopy (HR-EELS). Concurrent density functional theory calculations revealed many electronic structures from n-type semiconductors to metallic and p-type semiconducting nature with band bowing effect. The alloying-induced Peierls distortion leads to a change in crystal symmetry and decreased interlayer coupling. These alloys undergo indirect to direct bandgap transition with the function of Re concentration. These unique correlated structural and electronic properties of these 2D alloys can be potentially applicable for various electronic and optoelectronic devices.

Longitudinal acoustic and higher-energy excitations in the liquid phase-change materialGe2Sb2Te5

The dynamic structure factor $S(Q,E)$ of liquid ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$ has been obtained by inelastic x-ray scattering, where $Q$ and $E$ are momentum and energy transfer, respectively. The dispersion curve of the longitudinal acoustic excitation energy exhibits flat-topped $Q$ dependence similarly to that in liquid GeTe and liquid Bi, where the local structure is modulated by Peierls distortion. The flat-topped energy in liquid ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$ is split into low and high energy parts arising from Sb-Te and Ge-Te correlations, respectively. Furthermore, the high energy part depends on $Q$ like an optical mode with decreasing $Q$ to zero, and it approaches the vibrational energy of fourfold coordinated Ge atoms with octahedral configurations. The result indicates that a majority of Ge in the liquid stays in octahedral order as predicted by first-principles molecular dynamics simulations.

Enhanced Electron Heat Conduction in TaS3 1D Metal Wire.

The 1D wire TaS3 exhibits metallic behavior at room temperature but changes into a semiconductor below the Peierls transition temperature (Tp), near 210 K. Using the 3ω method, we measured the thermal conductivity of TaS3 as a function of temperature. Electrons dominate the heat conduction of a metal. The Wiedemann–Franz law states that the thermal conductivity of a metal is proportional to the electrical conductivity σ with a proportional coefficient of L0, known as the Lorenz number—that is, . Our characterization of the thermal conductivity of metallic TaS3 reveals that, at a given temperature T, the thermal conductivity κ is much higher than the value estimated in the Wiedemann–Franz (W-F) law. The thermal conductivity of metallic TaS3 was approximately 12 times larger than predicted by W-F law, implying . This result implies the possibility of an existing heat conduction path that the Sommerfeld theory cannot account for.

Orbital-selective Peierls phase in the metallic dimerized chain MoOCl2

Using ab initio density functional theory, here we systematically study the monolayer ${\mathrm{MoOCl}}_{2}$ with a $4{d}^{2}$ electronic configuration. Our main result is that an orbital-selective Peierls phase (OSPP) develops in ${\mathrm{MoOCl}}_{2}$, resulting in the dimerization of the Mo chain along the $b$ axis. Specifically, the Mo-${d}_{xy}$ orbitals form robust molecular-orbital states inducing localized ${d}_{xy}$ singlet dimers, while the Mo-${d}_{xz/yz}$ orbitals remain delocalized and itinerant. Our study shows that ${\mathrm{MoOCl}}_{2}$ is globally metallic, with the Mo-${d}_{xy}$ orbital bonding-antibonding splittings opening a gap and the Mo-${d}_{xz/yz}$ orbitals contributing to the metallic conductivity. Overall, the results resemble the recently much discussed orbital-selective Mott phase but with the localized band induced by a Peierls distortion instead of Hubbard interactions. Finally, we also qualitatively discuss the possibility of OSPP in the $3{d}^{2}$ configuration, as in ${\mathrm{CrOCl}}_{2}$.

Comment on "Spin-Lattice Coupling and the Emergence of the Trimerized Phase in the S=1 Kagome Antiferromagnet Na_{2}Ti_{3}Cl_{8}".

Recently the structural transition in Na2Ti3Cl8 at 200 K with the formation of triangular Ti3 clusters was studied by ab-initio calculations in PRL 124, 167203 (2020). It was concluded on the basis of the DFT+U results that the usual band effects are not sufficient to explain this transition. The authors then invoked magnetic mechanisms, including complicated factors such as biquadratic and ring exchanges. We point out that this overcomplicated description is unnecessary, and that the formation of trimers, or breathing kagome lattice in Na2Ti3Cl8, and in many other systems, has very simple, almost trivial explanation basing on the orbital-driven Peierls transition.

Self-organized topological insulator due to cavity-mediated correlated tunneling

Topological materials have potential applications for quantum technologies. Non-interacting topological materials, such as e.g., topological insulators and superconductors, are classified by means of fundamental symmetry classes. It is instead only partially understood how interactions affect topological properties. Here, we discuss a model where topology emerges from the quantum interference between single-particle dynamics and global interactions. The system is composed by soft-core bosons that interact via global correlated hopping in a one-dimensional lattice. The onset of quantum interference leads to spontaneous breaking of the lattice translational symmetry, the corresponding phase resembles nontrivial states of the celebrated Su-Schriefer-Heeger model. Like the fermionic Peierls instability, the emerging quantum phase is a topological insulator and is found at half fillings. Originating from quantum interference, this topological phase is found in "exact" density-matrix renormalization group calculations and is entirely absent in the mean-field approach. We argue that these dynamics can be realized in existing experimental platforms, such as cavity quantum electrodynamics setups, where the topological features can be revealed in the light emitted by the resonator.

Lattice melting and superconductivity in a group IV-VI compound

Inspired by the rich physical properties of IV-VI compounds, we choose polycrystalline ${\mathrm{Pb}}_{0.99}{\mathrm{Cr}}_{0.01}\mathrm{Se}$ to investigate its structural, vibrational, and electrical transport properties under pressure up to 50 GPa. The structural transitions from the $B1$ to $Pnma$ phase and then to the $B2$ phase in this sample are verified by the x-ray diffraction and Raman scattering measurements. The formation of the intermediate phase is suggested to be mediated by Peierls distortion, and the broad hump in the temperature-dependent resistivity in the intermediate phase gives further evidence of this phenomenon. When the material evolves into the $B2$ phase, superconductivity is observed to emerge, accompanied by suppressing the broad hump of resistivity at intermediate temperatures. Meanwhile, Hall coefficient measurements indicate that the carrier type changes during the structural transitions. These results suggest that the superconductivity in the $B2$ phase for this material is originated by ``melting'' the Peierls lattice distortion. By extending the present findings to other similar IV-VI semiconductors, we propose that all group IV-VI compounds could exhibit superconductivity in their $B2$ phase due to the lattice melting at high pressures.

Non-equilibrium charge density wave ground state of quasi-two-dimensional rare-earth tritelluride TbTe3

We have studied the time dependence of the relaxation of the non-equilibrium charge density wave (CDW) toward an equilibrium ground state in TbTe3 when the sample is cooled through the Peierls transition temperature under an electric field down to a given temperature, Texp. We show that when cooled at zero electric field or a value less than the threshold one, Et, for depinning the CDW at Texp, the CDW equilibrium ground state has a single phase in the sample volume. When cooled with an electric field higher than Et, the CDW ground state is in a frozen glass state, which can be destabilized only by reducing the electric field below Et. We tentatively interpret these results by the interaction of the CDW with a well ordered Te–Te discommensuration network.

Two-dimensional ferroelasticity in van der Waals \u03b2\u2019-In2Se3

Two-dimensional (2D) materials exhibit remarkable mechanical properties, enabling their applications as flexible and stretchable ultrathin devices. As the origin of several extraordinary mechanical behaviors, ferroelasticity has also been predicted theoretically in 2D materials, but so far lacks experimental validation and investigation. Here, we present the experimental demonstration of 2D ferroelasticity in both exfoliated and chemical-vapor-deposited β’-In2Se3 down to few-layer thickness. We identify quantitatively 2D spontaneous strain originating from in-plane antiferroelectric distortion, using both atomic-resolution electron microscopy and in situ X-ray diffraction. The symmetry-equivalent strain orientations give rise to three domain variants separated by 60° and 120° domain walls (DWs). Mechanical switching between these ferroelastic domains is achieved under ≤0.5% external strain, demonstrating the feasibility to tailor the antiferroelectric polar structure as well as DW patterns through mechanical stimuli. The detailed domain switching mechanism through both DW propagation and domain nucleation is unraveled, and the effects of 3D stacking on such 2D ferroelasticity are also discussed. The observed 2D ferroelasticity here should be widely available in 2D materials with anisotropic lattice distortion, including the 1T’ transition metal dichalcogenides with Peierls distortion and 2D ferroelectrics such as the SnTe family, rendering tantalizing potential to tune 2D functionalities through strain or DW engineering.

Pressure-induced structural transitions triggering dimensional crossover in the lithium purple bronze Li0.9Mo6O17

At ambient pressure, lithium molybdenum purple bronze (Li0.9Mo6O17) is a quasi-one dimensional solid in which the anisotropic crystal structure and the linear dispersion of the underlying bands produced by electronic correlations possibly bring about a rare experimental realization of Tomomaga-Luttinger liquid physics. It is also the sole member of the broader purple molybdenum bronzes family where a Peierls instability has not been identified at low temperatures. The present study reports a pressure-induced series of phase transitions between 0 and 12 GPa. These transitions are strongly reflected in infrared spectroscopy, Raman spectroscopy, and x-ray diffraction. The most dramatic effect seen in optical conductivity is the metallization of the c-axis, concomitant to the decrease of conductivity along the b-axis. This indicates that high pressure drives the material away from its quasi-one dimensional behavior at ambient pressure. While the first pressure-induced structure of the series is resolved, the identification of the underlying mechanisms driving the dimensional change in the physics remains a challenge.

Basic aspects of the metal–insulator transition in vanadium dioxide VO2: a critical review

Vanadium dioxide exhibits a first order metal to insulator transition (MIT) at 340 K (T MI ) from a rutile (R) structure to a monoclinic (M 1 ) structure. The mechanism of this transition interpreted as due either to a Peierls instability or to a Mott–Hubbard instability is controversial since half a century. However, in the last twenty years the study of chemical and physical properties of VO 2 and of its alloys, benefits of a renewed interest due to possible applications coming from the realization of devices made of thin films. We describe in this review the structural, electronic and magnetic properties of the different metallic (R) and insulating (M 1 , T, M 2 ) phases of VO 2 , of its solid solutions and under constraint. We present in a synthetic manner the various phase diagrams and their symmetry analysis. This work allows us to revisit older interpretation and to emphasize in particular the combined role of electron–electron interactions in the various phase of VO 2 and of structural fluctuations in the MIT mechanism. In this framework we show that the phase transition is surprisingly announced by anisotropic one-dimensional (1D) structural fluctuations revealing chain like correlations between the V due to an incipient instability of the rutile structure. This leads to an unexpected critical dynamics of the order–disorder (or relaxation) type. We describe how the two-dimensional (2D) coupling between these 1D fluctuations, locally forming uniform V 4+ zig-zag chains and V–V pairs, stabilizes the M 2 and M 1 insulating phases. These phases exhibit a 1D electronic anisotropy where substantial electron–electron correlations conduct to a spin–charge decoupling. The spin-Peierls ground state of M 1 is analyzed via a mechanism of dimerization, in the T phase, of the spin 1/2 V 4+ zig-zag Heisenberg chains formed in the M 2 phase. This review summarizes in a critical manner the main results of the large literature on fundamental aspects of the MIT of VO 2 . It is completed by unpublished old results. Interpretations are also placed in a large conceptual frame which is also relevant to interpret physical properties of other classes of materials.

Janus two-dimensional transition metal dichalcogenide oxides: First-principles investigation of WXO monolayers with X=S , Se, and Te

Structural symmetry breaking in two-dimensional materials can lead to superior physical properties and introduce an additional degree of piezoelectricity. In the present paper, we propose three structural phases ($1H, 1T$, and $1{T}^{\ensuremath{'}}$) of Janus $\mathrm{W}X\mathrm{O}$ ($X=\mathrm{S}$, Se, and Te) monolayers and investigate their vibrational, thermal, elastic, piezoelectric, and electronic properties by using first-principles methods. Phonon spectra analysis reveals that while the $1H$ phase is dynamically stable, the $1T$ phase exhibits imaginary frequencies and transforms to the distorted $1{T}^{\ensuremath{'}}$ phase. Ab initio molecular dynamics simulations confirm that $1H$- and $1{T}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{W}X\mathrm{O}$ monolayers are thermally stable even at high temperatures without any significant structural deformations. Different from binary systems, additional Raman active modes appear upon the formation of Janus monolayers. Although the mechanical properties of $1H\text{\ensuremath{-}}\mathrm{W}X\mathrm{O}$ are found to be isotropic, they are orientation dependent for $1{T}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{W}X\mathrm{O}$. It is also shown that $1H\text{\ensuremath{-}}\mathrm{W}X\mathrm{O}$ monolayers are indirect band-gap semiconductors and the band gap narrows down the chalcogen group. Except $1{T}^{\ensuremath{'}}$-WSO, $1{T}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{W}X\mathrm{O}$ monolayers have a narrow band gap correlated with the Peierls distortion. The effect of spin-orbit coupling on the band structure is also examined for both phases and the alteration in the band gap is estimated. The versatile mechanical and electronic properties of Janus $\mathrm{W}X\mathrm{O}$ monolayers together with their large piezoelectric response imply that these systems are interesting for several nanoelectronic applications.

Measurements of nonequilibrium interatomic forces using time-domain x-ray scattering

We determine experimentally the excited-state interatomic forces in photoexcited bismuth. The forces are obtained by a constrained least-squares fit of the excited-state dispersion obtained by femtosecond time-resolved x-ray diffuse scattering to a fifteen-nearest neighbor Born-von Karman model. We find that the observed softening of the zone-center $A_{1g}$ optical mode and transverse acoustic modes with photoexcitation are primarily due to a weakening of three nearest neighbor forces along the bonding direction. This provides a more complete picture of what drives the partial reversal of the Peierls distortion previously observed in photoexcited bismuth.

Dual Orbital Degeneracy Lifting in a Strongly Correlated Electron System.

The local structure of NaTiSi_{2}O_{6} is examined across its Ti-dimerization orbital-assisted Peierls transition at 210K. An atomic pair distribution function approach evidences local symmetry breaking preexisting far above the transition. The analysis unravels that, on warming, the dimers evolve into a short range orbital degeneracy lifted (ODL) state of dual orbital character, persisting up to at least 490K. The ODL state is correlated over the length scale spanning ∼6 sites of the Ti zigzag chains. Results imply that the ODL phenomenology extends to strongly correlated electron systems.

Peierls transition driven ferroelasticity in the two-dimensional d−f hybrid magnets

For broad nanoscale applications, it is crucial to implement more functional properties, especially those ferroic orders, into two-dimensional materials. Here GdI$_3$ is theoretically identified as a honeycomb antiferromagnet with large $4f$ magnetic moment. The intercalation of metal atoms can dope electrons into Gd's $5d$-orbitals, which alters its magnetic state and lead to Peierls transition. Due to the strong electron-phonon coupling, the Peierls transition induces prominent ferroelasticity, making it a multiferroic system. The strain from undirectional stretching can be self-relaxed via resizing of triple ferroelastic domains, which can protect the magnet aganist mechnical breaking in flexible applications.

Metal-semiconductor transition in the supercooled liquid phase of the Ge2Sb2Te5 and GeTe compounds

The ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$ and GeTe compounds are of interest for applications in phase change memories. In the reset process of the memory the crystal is rapidly brought above the melting temperature ${T}_{m}$ by Joule heating and then the liquid phase rapidly cools down leading to the formation of the amorphous phase. Since the liquid above ${T}_{m}$ is metallic and the amorphous phase is semiconducting a semiconductor-to-metal transition occurs in the supercooled liquid. Based on density functional simulations, we estimated the metal-semiconductor transition temperature ${T}_{\text{M-SC}}$ by monitoring the opening of a band gap in the supercooled liquid phase. Due to previous evidence on the importance of the van der Waals (vdW) interaction in describing the liquid phase of these materials, we used both the revised Vydrov-van Voorhis functional which includes vdW nonlocal interactions and the Perdew-Burke-Ernzerhof functional without vdW corrections. The estimated ${T}_{\text{M-SC}}$ is about 100--150 K higher with the former than with the latter framework for both compounds. By including vdW interactions the estimated ${T}_{\text{M-SC}}$ is closer to ${T}_{m}$ than to the glass transition temperature for both systems. The analysis of the structural properties as a function of temperature suggests a correlation between the metal-semiconductor transition and a Peierls distortion. However, the data support more a continuous structural transformation than the presence of a first order liquid-liquid phase change associated the metal-semiconductor transition.

Resistivity of Cu, Ni and Sb in undercooled liquid state

With the help of non-contact measurement, electrical resistivity of Cu, Ni, Sb has been measured to study the influence of short range order on electrons scattering in the normal and undercooled liquid state. Till the maximum undercooling, linear resistivity-temperature dependence is obtained in Cu and Ni melts. Nonlinear resistivity behavior is obtained in undercooled liquid Sb till a maximum undercooling of about 60 degree. The increase in viscous flow activation energy reveals structure transition in Sb melts. A drop in resistivity of Cu melts is obtained accompanying the recalescence. The abrupt jump in resistivity of Sb reveals the rapid growth of Peierls distortion during rapid solidification process.